Patient ventilators, that is, devices for controlling or assisting the breathing of a patient, are known in the art, and an embodiment of such a ventilator which is particularly compatible with the improvement disclosed herein is described in U.S. Pat. application Ser. No. 401,739, entitled Patient Ventilator Apparatus, filed on Sept. 28, 1973, in the name of George K. Russell. In that apparatus, a desired oxygen-air mixture is supplied under pressure to a patient, in a specific desired quantity, and in desired inhale/exhale half-cycles. For example, the ventilator apparatus may be controlled by a master flip flop circuit, pneumatically or electrically operated, so that both the inspiratory and expiratory half-cycles can be initiated by manual triggers and by timing circuit triggers, and wherein the inspiratory half-cycle can be triggered also by a detector device which senses an attempt by the patient to inhale, while the expiratory half-cycle can be triggered also by a volume limit sensor and a pressure limit sensor.
In the use of such apparatus for controlling the breathing of patients, it is often necessary to "wean" the patient from dependence upon the apparatus, and for this purpose the apparatus is adjusted so that the number of mandatory breathing cycles applied to the patient are slowly reduced until the patient is able to breath on his own. This concept is referred to as "intermittent mandatory ventilation" (IMV).
In the use of the apparatus in this manner, it is preferable to ensure that each mandatory breath supplied to the patient is initiated when the patient has concluded a voluntary expiratory half-cycle, so that voluntary exhalations of air are not interrupted. This concept will be referred to as synchronized intermittent mandatory ventilation (SIMV), and an object of the instant invention is to provide circuitry for accomplishing this synchronized function.